This invention relates to a circuit for generating an error in a control loop for an IF combiner, and more particularly to a circuit capable of providing a broad-band quadrature phase-modulated signal for such a control loop.
The present invention finds particular application in a high density space diversity radio system. Such a system is used to improve reliability in a microwave radio, as a solution to the problem of frequency selective fading due to multipath propagation. Reliability is improved by combining signals received by separate antennas. Circuitry according to the present invention can be used to control the phase of IF signals from two different receivers with respect to each other, so that the two signals can be combined in phase or in accordance with other combining strategies.
A prior art combiner employs a common local oscillator for two down-converters and space diversity receivers. The local oscillator output going to the down-converter which is to be phase shifted is controlled by a narrow-band phase shifter. A control loop senses the output of the combiner and controls the phase shifter. A small amount of phase modulation is introduced into the message channel so as to produce an amplitude modulation from which the control loop can detect the phase error between main and diversity receivers. This technique uses band-sensitive and expensive radio frequency components.
Another prior method of phase shifting employs side-step conversion on an IF signal. This method is similar to the previously described one, except that it uses an up-converter and a down-converter. The local oscillator for the side-step conversion is split and, as in the prior technique, one side is narrow-band phase shifted. This method can be used at a common IF frequency, regardless of the R.F. band of the receiver. Unfortunately, the method gives rise to high spurious products in the IF output and is extremely bad for high density FM receivers.
These prior art IF combiners have a number of limitations. Importantly, as a result of their operation, a small modulation is introduced into the message channel.
Finally, since the modulation used to permit detection of an error signal in the prior art methods is introduced into the message channel, it can only be a very small modulation. This limits the sensitivity of the error detection process.